Create Polygon with color of border is different from inside


I would like to draw a polygon that border's color is not same as inside's color as below:
Link image

I think this it can be achieved by combining FillPolygon and DrawPolygon (create border).
However, DrawPolygon is not available in WriteableBitmapEx.

I tried to use DrawPolyline, but the result is not good, the polygon created by FillPolygon and DrawPolyline are not same.

Do you have any suggestion for me?
Thank you very much!